In a telephone system, call progress tones are used for indicating to the user the status of his call. Typical signals are labelled audible ringing, dial tone, busy tone, and reorder tone. In the conventional system, these tones are detectable by the user of the system. However, the creation of private networks has resulted in the need for circuitry capable of recognizing these call progress tones since these may not be available at the call originating end of such networks. In addition, since these tones are the basis for billing procedures on these networks, the detection circuits need to be accurate and fast so that the billing period is accurately identified. Furthermore, the proliferation of digital switching systems has created the additional requirement that any such detection circuitry be able to operate directly on pulse code modulated (PCM) signals.
Unfortunately, the North American telephone network does not have a precise tone plan that has been universally adopted. There is therefore a considerable variation in the tone frequencies and cadence employed by various telephone utilities. A comprehensive listing of the various tones may be found in the publication entitled "Notes on the Network" published in 1980 by the American Telephone and Telegraph Company at pages 110 to 119. The most common of the call progress tones are dial tone, audible ringing tone, busy tone, and reorder tone. The precise tone plan specification for the North American network defines these tones as follows, dial tone is a continuous tone having frequencies of 350 and 440 Hz at a level of -13 dbm. Audible ringing tone is defined as comprising frequencies of 440 and 480 Hz at a level of -19 dbm and a cadence of 2 seconds ON and 4 seconds OFF. Busy tone is defined as having frequency components of 480 and 620 Hz at a level of -24 dbm and a cadence of half a second ON and half a second OFF, whereas reorder tone contains the same frequency components at a comparable level but with a cadence of 0.25 of a second ON and 0.25 of a second OFF. Other frequencies and levels are generated by older equipment which does not follow the precise tone plan; these tones are either formed as dual frequency tones or as amplitude modulated signals where a higher frequency is modulated by a lower frequency.
It is therefore desired to provide a circuit for the automatic recognition and identification of call progress tones on the telephone network which is accurate, reliable, fast and economical. It is further desired to provide such a circuit that operates on digital signals.
In the past, there has been a wide variety of methods proposed for solving this problem including digital filtering and spectral analysis of the signals. More recently, there have been proposals that provide somewhat simpler circuitry to recognize the basic call progress tones. The first of these is known as the energy system and is based on the recognition of energy bursts in the signal, whereby counting the number of "tone bursts" of the signal in the specified period indicates the identity of the tone. Effectively, this method relies on the detection of the cadence of the signal. A second method, relies on the incidence of zero crossings by the signal being detected. However, the known methods of detection have been found to be inadequate especially in the identification of dual-frequency tones.